Substrate-guide optical device

ABSTRACT

A method fabricates an optical device that includes at least one partially reflecting surface and a light-wave transmitting substrate is fabricated. A plurality of transparent plates is obtained. At least one surface of at least one of the transparent plates is coated with a partially reflecting coating to form the at least one partially reflecting surface. The plates are arranged in a stack and attached to each other. The plates at the opposing ends of the stack are thicker than the remaining plates in the stack. The stack is sliced to form the substrate such that the substrate has two major external surfaces. The substrate is grinded or polished, and at least one of the two major external surfaces is coated with a coating that compensates for non-uniformity of the substrate.

FIELD OF THE INVENTION

The present invention relates to substrate-guided optical devices, andparticularly to devices which include a plurality of anisotropicreflecting surfaces carried by a light-transmissive substrate, alsoreferred to as a light wave-guide optical element (LOE).

The invention can be implemented to advantage in a large number ofimaging applications, such as, for example, head-mounted and head-updisplays, cellular phones, compact displays, 3-D displays, compact beamexpanders as well as non-imaging applications such as flat-panelindicators, compact illuminators and scanners.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is inhead-mounted displays wherein an optical module serves both as animaging lens and a combiner, in which a two-dimensional display isimaged to infinity and reflected into the eye of an observer. Thedisplay can be obtained directly from either a spatial light modulator(SLM) such as a cathode ray tube (CRT), a liquid crystal display (LCD),an organic light emitting diode array (OLED), or a scanning source andsimilar devices, or indirectly, by means of a relay lens or an opticalfiber bundle. The display comprises an array of elements (pixels) imagedto infinity by a collimating lens and transmitted into the eye of theviewer by means of a reflecting or partially reflecting surface actingas a combiner for non-see-through and see-through applications,respectively. Typically, a conventional, free-space optical module isused for these purposes. Unfortunately, as the desired field-of-view(FOV) of the system increases, such a conventional optical modulebecomes larger, heavier, bulkier and therefore, even for a moderateperformance device, impractical. This is a major drawback for all kindsof displays but especially in head-mounted applications, wherein thesystem must necessarily be as light and as compact as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on one hand, are still not sufficiently compactfor most practical applications, and, on the other hand, suffer majordrawbacks in terms of manufacturability. Furthermore, the eye-motion-boxof the optical viewing angles resulting from these designs is usuallyvery small—typically less than 8 mm. Hence, the performance of theoptical system is very sensitive, even to small movements of the opticalsystem relative to the eye of the viewer, and do not allow sufficientpupil motion for conveniently reading text from such displays.

SUMMARY OF THE INVENTION

The present invention facilitates the design and fabrication of verycompact LOEs for, amongst other applications, head-mounted displays. Theinvention allows relatively wide FOV's together with relatively largeeye-motion-box values. The resulting optical system offers a large,high-quality image, which also accommodates large movements of the eye.The optical system offered by the present invention is particularlyadvantageous because it is substantially more compact than state-of-theart implementations and yet it can be readily incorporated, even intooptical systems having specialized configurations.

The invention also enables the construction of improved head-up displays(HUDs). Since the inception of such displays more than three decadesago, there has been significant progress in the field. Indeed, HUDs havebecome popular and they now play an important role, not only in mostmodem combat aircraft, but also in civilian aircraft, in which HUDsystems have become a key component for low visibility landingoperation. Furthermore, there have recently been numerous proposals anddesigns for HUDs in automotive applications where they can potentiallyassist the driver in driving and navigation tasks. Nevertheless,state-of the-art HUDs suffer several significant drawbacks. All HUDs ofthe current designs require a display source that must be offset asignificant distance from the combiner to ensure that the sourceilluminates the entire combiner surface. As a result, thecombiner-projector HUD system is necessarily bulky, and large, andrequires considerable installation space, which makes it inconvenientfor installation and at times even unsafe to use. The large opticalaperture of conventional HUDs also poses a significant optical designchallenge, either rendering the HUDs with compromising performance, orleading to high cost wherever high-performance is required. Thechromatic dispersion of high-quality holographic HUDs is of particularconcern.

An important application of the present invention relates to itsimplementation in a compact HUD, which alleviates the aforementioneddrawbacks. In the HUD design of the current invention, the combiner isilluminated with a compact display source that can be attached to thesubstrate. Hence, the overall system is very compact and can be readilyinstalled in a variety of configurations for a wide range ofapplications. In addition, the chromatic dispersion of. the display isnegligible and, as such, can operate with wide spectral sources,including a conventional white-light source. In addition, the presentinvention expands the image so that the active area of the combiner canbe much larger than the area that is actually illuminated by the lightsource.

A further application of the present invention is to provide a compactdisplay with a wide FOV for mobile, hand-held application such ascellular phones. In today's wireless internet-access market, sufficientbandwidth is available for full video transmission. The limiting factorremains the quality of the display within the device of the end-user.The mobility requirement restricts the physical size of the displays,and the result is a direct-display with poor image viewing quality. Thepresent invention enables a physically very compact display with a verylarge virtual image. This is a key feature in mobile communications, andespecially for mobile internet access, solving one of the mainlimitations for its practical implementation. Thereby the presentinvention enables the viewing of the digital content of a full formatinternet page within a small, hand-held device, such as a cellularphone.

A broad object of the present invention is therefore to ameliorate thedrawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

In accordance with the present invention there is provided an opticaldevice, comprising a light waves-transmitting substrate having at leasttwo major surfaces and edges, optical means for coupling light wavesinto said substrate by total internal reflection, and a plurality ofpartially reflecting surfaces carried by said substrate wherein saidpartially reflecting surfaces are parallel to each other and are notparallel to any of the edges of said substrate, characterized in that atleast one of said partially reflecting surfaces is an anisotropicsurface.

BRIEF DESCRIPTION OF THE DRAWINGS

The exact nature of this invention, as well as the objects andadvantages thereof, will become readily apparent from consideration ofthe following specification in conjunction with the accompanyingdrawings in which like reference numerals designate like partsthroughout the figures thereof and wherein:

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention. The description taken with the drawings are to serve asdirection to those skilled in the art as to how the several forms of theinvention may be embodied in practice.

FIG. 1 is a side view of a generic form of a prior art folding opticaldevice;

FIG. 2 is a side view of an exemplary LOE, in accordance with thepresent invention;

FIGS. 3A and 3B illustrate the reflectance and the transmittanceperformance of an anisotropic reflecting surface which is oriented torespectively reflect s-polarized light waves and p-polarized lightwaves;

FIG. 4 illustrates a detailed sectional view of an exemplary array ofselectively reflective surfaces;

FIG. 5 illustrates a detailed sectional view of an exemplary array ofselectively reflective surfaces with a retardation plate;

FIG. 6 illustrates a detailed sectional view of an exemplary array ofselectively reflective surfaces with a second retardation plate attachedto the first surface of the substrate;

FIG. 7 is a schematic sectional-view of a reflective surface accordingto the present invention;

FIG. 8 is a diagram illustrating the polarization vector of the coupledwave and the major axis of a reflective surface;

FIG. 9 is a diagram illustrating the polarization vector of the coupledwave and the major axis of another reflective surface;

FIG. 10 is a schematic sectional-view of a reflective surface with twodifferent impinging rays according to the present invention;

FIG. 11 illustrates a sectional view of an exemplary array ofselectively reflective surfaces wherein a blank plate is attached to thesubstrate edge;

FIG. 12 illustrates the active aperture-size of the reflecting surfacesas a function of the field angle for an exemplary LOE;

FIG. 13 is a schematic sectional-view of a reflective surface accordingto the present invention illustrating the actual active aperture of thesurface;

FIG. 14 illustrates detailed sectional views of the reflectance from anexemplary array of selectively reflective surfaces, for three differentviewing angles;

FIG. 15 illustrates detailed sectional views of the reflectance from anexemplary array of selectively reflective surfaces, for two differentviewing angles;

FIG. 16 illustrates the required distance between two adjacentreflecting surfaces as a function of the field angle for an exemplaryLOE;

FIG. 17 illustrates a sectional view of an exemplary array ofselectively reflective surfaces wherein a wedged blank plate is attachedto the substrate edge;

FIG. 18 is a diagram illustrating steps (a) to (e) of a method forfabricating an array of partially reflecting surfaces according to thepresent invention;

FIG. 19 is a diagram illustrating steps (a) to (e) of another method forfabricating an array of partially reflecting surfaces according to thepresent invention;

FIG. 20 is a diagram illustrating steps (a) to (e) of a modified methodfor fabricating an array of partially reflecting surfaces according tothe present invention;

FIG. 21 is a diagram illustrating steps (a) and (b) of a method toattach a blank plate at the edge of the LOE;

FIG. 22 is a diagram illustrating steps (a) and (b) in a further methodfor fabricating an array of partially reflecting surfaces, according tothe present invention;

FIG. 23 is a diagram illustrating steps (a) and (b) of still a furthermethod for fabricating an array of partially reflecting surfaces,according to the present invention,

FIG. 24 is a schematic sectional˜view of a reflective surface embeddedinside an LOE;

FIG. 25 is a diagram illustrating steps (a) and (b) of a method forfabricating an array of partially reflecting surfaces along with acoupling-in reflecting surface according to the present invention;

FIG. 26 illustrates an exemplary ray which is coupled into an LOE systemby a coupling-in prism;

FIG. 27 illustrates an exemplary embodiment of an LOE embedded in astandard eyeglass frames;

FIG. 28 illustrates an exemplary embodiment of an LOE embedded in astandard eyeglass frames wherein a video camera is attached to theeyeglass frames;

FIG. 29 illustrates an exemplary embodiment of an LOE embedded in a handcarried display system, and

FIG. 30 illustrates an exemplary HUD system in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a prior art folding optics arrangement, wherein thesubstrate 2 is illuminated by a display source 4. The display iscollimated by a collimating lens 6. The light from the display source 4is coupled into substrate 2 by a first reflecting surface 8, in such away that the main ray 10 is parallel to the substrate plane. A secondreflecting surface 12 couples the light waves out of the substrate andinto the eye 14 of a viewer. Despite the compactness of thisconfiguration, it suffers significant drawbacks; in particular only avery limited FOV can be affected. As shown in FIG. 1, the maximumallowed off-axis angle inside the substrate is:

$\begin{matrix}{{\alpha_{\max} = {\arctan\left( \frac{T - d_{eye}}{2l} \right)}},} & (1)\end{matrix}$wherein T is the substrate thickness;

d_(eye) is the desired exit-pupil diameter, and

l is the distance between reflecting surfaces 8 and 12.

With angles higher than α_(max). the rays are reflected from thesubstrate surface before arriving at the reflecting surface 12. Hence,the reflecting surface 12 will be illuminated at an undesired directionand ghost images will appear.

Therefore, the maximum achievable FOV with this configuration is:FOV_(max)≈2vα _(max),  (2)wherein v is the refractive index of the substrate.

Typically the refractive index values lie in the range of 1.5-1.6.

Commonly, the diameter of the eye pupil is 2-6 mm. To accommodatemovement or misalignment of the display, a larger exit-pupil diameter isnecessary. Taking the minimum desirable value at approximately 8-10 mm,the distance between the optical axis of the eye and the side of thehead, l, is, typically, between 40 and 80 mm. Consequently, even for asmall FOV of 8°, the desired substrate thickness would be of the orderof 12 mm.

Methods have been proposed to overcome the above problem. These include,utilizing a magnifying telescope inside the substrate and non-parallelcoupling directions. Even with these solutions however, and even if onlyone reflecting surface is considered, the system thickness remainslimited by a similar value. The FOV is limited by the diameter of theprojection of the reflective surface 12 on the substrate plane.Mathematically, the maximum achievable FOV, due to this limitation, isexpressed as:

$\begin{matrix}{{{F\; O\; V_{\max}} \approx \frac{{T\;\tan\;\alpha_{sur}} - d_{eye}}{R_{eye}}},} & (3)\end{matrix}$wherein, α_(sur), is the angle between the reflecting surface and thenormal to the substrate plane, and R_(eye), is the distance between theeye of the viewer and the substrate (typically, about 30-40 nun).

In practice, tan α_(sur) cannot be much larger than 1; hence, for thesame parameters described above for a FOV of 8°, the required substratethickness here is of the order of 7 mm, which is an improvement on theprevious limit. Nevertheless, as the desired FOV is increased, thesubstrate thickness increases rapidly. For instance, for desired FOVs of15° and 30° the substrate limiting thickness is 18 mm and 25 mm,respectively.

To alleviate the above limitations, the present invention utilizes anarray of selectively reflecting surfaces, fabricated within an LOE. FIG.2 illustrates a sectional view of an LOE according to the presentinvention. The first reflecting surface 16 is illuminated by acollimated display 18 emanating from a light source (not shown) locatedbehind the device. The reflecting surface 16 reflects the incident lightfrom the source such that the light waves are trapped inside a planarsubstrate 20 by total internal reflection. After several reflections offthe surfaces of the substrate, the trapped light waves reach an array ofselectively reflecting surfaces 22, which couple the light waves out ofthe substrate into the eye 24 of a viewer. Assuming that the centralwave of the source is coupled out of the substrate 20 in a directionnormal to the substrate surface 26, the reflecting surfaces 22 are flat,and the off-axis angle of the coupled wave inside the substrate 20 isα_(in), then the angle α_(sur2) between the reflecting surfaces and thenormal to the substrate plane is:

$\begin{matrix}{\alpha_{{sur}\; 2} = {\frac{\alpha_{i\; n}}{2}.}} & (4)\end{matrix}$

As can be seen in FIG. 2, the trapped rays arrive at the reflectingsurfaces from two distinct directions 28, 30. In this particularembodiment, the trapped rays arrive at the reflecting surface from oneof these directions 28 after an even number of reflections from thesubstrate front and back surfaces 26, wherein the incident angle β_(ref)between the trapped ray and the normal to the reflecting surface is:

$\begin{matrix}{\beta_{ref} = {{{90{^\circ}} - \left( {\alpha_{i\; n} - \alpha_{{sur}\; 2}} \right)} = {{90{^\circ}} - {\frac{\alpha_{i\; n}}{2}.}}}} & (5)\end{matrix}$

The trapped rays arrive at the reflecting surface from the seconddirection 30 after an odd number of reflections from the substratesurfaces 26, where the off-axis angle is α′_(in)=180°−α_(in) and theincident angle between the trapped ray and the normal to the reflectingsurface is:

$\begin{matrix}{\beta_{ref}^{\prime} = {{{90{^\circ}} - \left( {\alpha_{i\; n}^{\prime} - \alpha_{{sur}\; 2}} \right)} = {{{90{^\circ}} - \left( {{180{^\circ}} - \alpha_{i\; n} - \alpha_{{sur}\; 2}} \right)} = {{{- 90}{^\circ}} + {\frac{3\;\alpha_{i\; n}}{2}.}}}}} & (6)\end{matrix}$

As illustrated in FIG. 2, for each reflecting surface, each ray firstarrives at the surface from the direction 30, wherein some of the raysimpinge on the surface again, from direction 28. In order to preventundesired reflections and ghost images, it is important that thereflectance be negligible for the rays that impinge on the surfacehaving the second direction 28. The desired discrimination between thetwo incident directions can be achieved by exploiting the fact that theundesired direction meets the surface after the ray has transferred thesurface in the desired direction. Two solutions to this requirement,both exploiting angular sensitivity of thin film coatings werepreviously proposed. An alternative solution is presently described,exploiting anisotropic reflecting surfaces, that is, optical surfaceshaving a major axis parallel to the surface plane wherein the reflectionand transmission properties of the surface at least partly depend on theorientation of the polarization of the incident light waves in relationto the major axis of the surface.

FIG. 3A illustrates an example of an anisotropic partially reflectingsurface acting as a beamsplitter 40, having a major axis 42. Anunpolarized light wave 44 impinges on the surface. The partiallyreflecting surface reflects the component of the light wave 46 with anelectrical field vector parallel to the major axis 42 and transmits thecomponent of the light wave 48 with an electrical field vectorperpendicular to the major axis 42.

A possible candidate for the required anisotropic partially reflectingelement can be a wire grid polarizing beamsplitter 40, generally in theform of an array of thin parallel conductors supported by a transparentsubstrate. The key factor that determines the performance of a wire gridpolarizing beamsplitter 40 is the relationship between thecenter-to-center spacing, or period, of the parallel grid elements andthe wavelength of the incident radiation. When the grid spacing orperiod is much shorter than the wavelength, the grid functions as apolarizing beamsplitter 40 that reflects electromagnetic radiationpolarized parallel to the grid elements. and transmits radiation of theorthogonal polarization. In this case, as illustrated in FIG. 3A, themajor axis of a wire grid polarizing beamsplitter 40 is defined asparallel to the array of conductors 49. Usually, in order to obtain thebest transmission and contrast, the wire grid polarizing beamsplitter 40should be used to transmit the p-polarization and reflect thes-polarization, as illustrated in FIG. 3A. However, as illustrated inFIG. 3B it is possible to use the beamsplitter also in the orthogonalorientation. That is, the main axis 52 of the polarizing beamsplitter 50is oriented parallel to the propagation direction of the incident beam44. Since now the major axis of the polarizing beamsplitter 50 isparallel to the electric field of the p-polarized light, the polarizingbeamsplitter 50 reflects the component of the p-polarized light waves 56with its electrical field vector parallel to the major axis 52 andtransmits the component of the s-polarized light 58 with its electricalfield vector perpendicular to the major axis 52. Usually, the geometryillustrated in FIG. 3A has reduced efficiency and contrast compared tothe one described in FIG. 3B. However, for some applications thisgeometry can also be useful.

FIG. 4 illustrates an example of an LOE, exploiting wire grid polarizingbeamsplitters as partially reflecting surfaces, according to the presentinvention. The first reflecting surface 16 is illuminated by anunpolarized collimated display 18 emanating from a light source (notshown) located behind the device. The reflecting surface 16 reflects theincident light waves from the source such that the light waves aretrapped inside a planar substrate 20 by total internal reflection. Afterseveral reflections off the surfaces of the substrate, the trapped lightwaves reach the first partially reflecting surface 22 a, the major axisof which is oriented to reflect the s-polarized component 60 of thetrapped wave 18. The p-polarized component 62 is transmitted through thereflecting surface 22 a and then reflected by the second reflectingsurface 22 b, the major axis of which is oriented to reflect p-polarizedlight waves. Both the s-polarized 60 and the p-polarized 62 light wavesare coupled out of the substrate into the eye of a viewer. Naturally,for this configuration the polarization of the input beam should beuniform. Hence, care must be taken to prevent birefringent effects fromthe collimating lens as well as in the substrate 20 itself.

An LOE with non-identical selectively reflecting surfaces has two issueswhich must be addressed. In see-through systems, such as head-mounteddisplays for pilots, wherein the viewer should see the external scenethrough the LOE, the transmittance of the selectively reflectingsurfaces should be relatively high. Since in this case the reflectancecoefficient is not identical for all of the selectively reflectingsurfaces, there is a danger that this would result in a non-uniformimage of the external scene viewed through the substrate. In thegeometry illustrated in FIG. 5, the transmittance of surfaces 22 a and22 b for the external light, is around 50% for both reflecting surfaces.Surface 22 a transmits the p-polarized component of the external light,while surface 22 b transmits the s-polarized component. In othersituations, where such potential non-uniformity is crucial, acomplementary non-uniform coating could be added on the external surfaceof the substrate, to compensate for the non-uniformity of the substrateand to achieve a view of uniform brightness over the entire FOV.

The other phenomenon that might result from the proposed configuration,is the polarization non-uniformity of both projected and externalscenes. For most of the potential applications, this non-uniformity doesnot cause any disturbance, but for some applications, the achievement ofuniformly polarized projected and external images over the entireaperture may be necessary. This non-uniformity can be corrected byinserting an optional retardation plate between the substrate and theeye. As illustrated in FIG. 5 a half-wavelength plate 64 is insertednext to the projection of the first reflecting surface 22 a on the frontsurface 26. As a result, the polarization of the reflecting wave 66 isrotated to p-polarization while the polarization of the transmittedexternal wave 67 is rotated to s-polarization. For the second surface 22b the reflecting wave 62 and the transmitted wave 68 remained p- ands-polarized respectively. As a result, the entire projected image isp-polarized while the entire image of the external scene is s-polarized.Naturally, it is possible to invert these polarizations by inserting thehalf-wavelength plate next to the second reflecting surface 22 b toyield an s-polarized projected image with a p-polarized external scene.

Another possible problem, which is related to see-through systems, isthe orientation of the major ‘axis of the second reflection surface 22a. As illustrated in FIG. 5 the polarizing beamsplitter is oriented toreflect p-polarized light waves. For impinging light waves 62 havingincident angles smaller than 30°, this might be sufficient to achievegood contrast. However, for higher incident angles the reflectingsurface 22 b might have reduced efficiency and contrast compared to thatof surface 22 a. As illustrated in FIG. 6, this problem might be solvedby inserting an off axis half-wave plate 69 in front of surface 22 b,and by setting the orientation of the major axis of surface 22 b to be.the same as surface 22 a. That is, wire grid polarizing beamsplitter isoriented to reflect the s-polarization and transfer the p-polarization.Therefore, the polarization of the beam 62 is rotated by 90° tos-polarization, before impinging on surface 22 b. The s-polarized lightwaves is then reflected with high efficiency and contrast from surface22 b and again rotated by 90° to be p-polarized. As can be seen in FIG.6, the external scene is s-polarized to the viewer's eye. However, onlythe p-polarized component of the external scene is transmitted throughthe reflecting surfaces 22 a and 22 b. This might be a problem forapplications where it is required to observe an s-polarized image in theexternal scene, such as an LCD display or the like. In these cases, itis possible to insert another retardation plate 65 next to the frontsurface 26. Thus, if a half-wavelength plate is inserted then only thes-polarized component light waves from the external scene are projectedinto the viewer's eye, and if a quarter-wave plate is inserted, thenhalf of the energy from the s-polarized as well as the p-polarized lightwaves are projected from the external scene.

In non-see-through systems, such as virtual-reality displays, thesubstrate can be opaque and the transmittance of the system has noimportance. In that case, it is possible to utilize a simple reflectingmirror instead of the second partially reflecting surface 22 b. Again,it is possible to insert a half-wavelength plate next to one of thereflecting surface to achieve a uniformly polarized projected image.

FIG. 7 is a detailed sectional view of the selectively reflectivesurface 22 a, which couples light waves trapped inside the substrateout, and into the eye of a viewer. As can be seen, in each cycle thecoupled ray 18 passes through reflecting surfaces 22 a three times. Atthe first incident point 70, the s-polarized component 72 is reflectedand coupled out of the substrate. The transmitted p-polarized component74 is reflected off the outer surfaces 26 and then meets surface 22 aagain at points 76 and 78. At these points, however, the incident lightwaves are p-polarized, while the orientation of surface 22 a is set toreflect s-polarized light waves and to transmit p-polarized light waves.Hence, the reflections at these points can be negligible, as requiredabove, in relation to FIG. 2, in order to prevent undesired reflectionsand ghost images.

Naturally, this concept can be valid not only for the exampleillustrated in FIGS. 4, 5 and 6 but also in more general cases, whereinmore than two partially reflecting surfaces are utilized to project theimage into the eyes of a viewer.

Referring now to FIG. 8, assuming a system wherein s-polarized lightwaves are coupled inside the substrate and the major axis of the firstreflecting surface is orientated at an angle β₁ to the projection of thepropagation direction of the coupled beam in the plane of the reflectingsurface, the angle between the major axis 80 and the electric field ofthe coupled wave 82 is 90°-β₁. The coupled light can be separated intotwo components, one 84, which is orthogonal to the major axis 80 andtransmitted through the surface 22 a, and the second 86, which isparallel to the major axis 80 and is reflected by surface 22 a at thefirst incident of the light at this surface. Since after the firstreflection from surface 22 a, only the orthogonal component 84 istransmitted and continues to propagate inside the substrate, at thefollowing incident points (if any) at surface 22 a, the reflectance ofthe coupled wave is negligible. Assuming that the initial intensity ofthe coupled radiation is 10, the intensities of the reflected and thetransmitted components are:I ₀ ^(r) =I ₀·sin²(β₁)  (7)andI ₀ ^(t) =I ₀·cos²(β₁)  (8)respectively.

As illustrated in FIG. 9, in order to enable reflectance of the coupledwave from the second surface 22 b, the major axis 90 of this surface isoriented at an angle β₂ to the projection of the propagation directionof the coupled wave in the plane of the reflecting surface. Assumingthat when passing through the wire grid polarizing beamsplitter theskew-ray transmitted polarization vector is not rotated in comparison tothe original polarization vector, the polarization of the coupled waveis oriented now at an angle β₁ to the s-polarization, the angle betweenthe major axis 90 and the electric field of the coupled wave 92 is90°-β₁−β₂. The intensity of the coupled wave before impinging on surface22 b is now:I ₁ =I ₀ ^(t) =I ₀·cos²(β₁)  (9)And the intensities of the transmitted and reflected components 94, 96are:I ₁ ^(t) =I ₁·cos²(β₁+β₂)  (10)andI ₁ ^(r) =I ₁·sin²(β₁+β₂)  (11)respectively.

Inserting Eq. (9) into Eqs. (10) and (11) yields:I ₁ ^(t) =I ₀·cos²(β₁)·cos²(β₁+β₂)  (12)andI ₁ ^(r) =I ₀·cos²(β₁)·sin²(β₁+β₂)  (13)

For systems where uniform brightness over the entire aperture isnecessary, the reflectance intensity from surfaces 22 a and 22 b shouldbe the same, that isI₁ ^(r)=I₀ ^(r).  (14)

Inserting Eqs. (7) and (13) into Eq. (14) yields:I ₀·cos²(β₁)·sin²(β₁+β₂)=I ₀·sin²(β₁)  (15)orsin(β₁+β₂)=tan(β₁).  (16)

Similarly, it is possible to calculate the orientation angle β_(n) ofeach of the plurality of the following reflecting surfaces 22 n, inorder to achieve a uniform brightness or any other required brightnessdistribution of the projected image. Usually, it is easier to startdesigning the orientation of the major axis of the reflecting surfacesfrom the last surface. Assuming that uniform brightness is required andthat the utilization of all the trapped energy inside the substrate isdesired, then the last surface should couple the entire coupled lightwaves out of the substrate. That is, the orientation of the major axisof the last surface should be parallel to the polarization of thetrapped light waves at that surface. Correspondingly, the surface beforethe last should reflect half of the incident wave, that is, theorientation of the major axis of this surface should be inclined at anangle of 45° with respect to the polarization of the incident wave atthis surface. In the same way, it is possible to calculate theorientation angles of the other surfaces and the required polarizationof the trapped wave 18. This design procedure might be modified byrotating the trapped light waves using retardation plates, similar tothat of surface 69 which is described above in reference to FIG. 6. Inaddition, the polarization of the waves might be rotated by attachingretardation plates to one or both of the external major surfaces of thesubstrate. In each case, the orientation of the major axis of eachreflecting surface should be set accordingly. Furthermore, during anydesign method, a possible rotation of the trapped light waves due to thetotal internal reflection from the external surfaces, or any loss due toabsorption in the reflecting surfaces or in the retardation plates,should be accounted for.

Another issue that should be considered is the actual active area ofeach reflecting surface. FIG. 10 illustrates a detailed sectional viewof the selectively reflective surface 22, which couples light wavestrapped inside the substrate out and into the eye of a viewer. As can beseen, the ray 18 a is reflected off the upper surface 26, next to theline 100, which is the intersection of the reflecting. surface 22 withthe upper surface 26. Since this ray does not impinge on the reflectingsurface 22, its polarization remains the same and its first incidence atsurface 22 is at the point 102, after double reflection from bothexternal surfaces. At this point, the wave is partially reflected andthe ray 104 is coupled out of the substrate. For other rays, such as ray18 b, which is located just below ray 18 a, the first incidence atsurface 22 is at point 106, before it meets the upper surface 26. Hence,when it is again incident on surface 22, at point 110 following doublereflection from the external surfaces, the polarization of the ray isnormal to that of the major axis of surface 22 and the reflectance thereis negligible. As a result, all the rays with the same coupled-in angleas 18 a that incident on surface 22 left to the point 102 are notreflected there. Consequently, surface 22 is actually inactive left ofthe point 102 for this particular couple-in angle.

Since the inactive portions of the selectively reflecting surfaces 22 donot contribute to the coupling of the trapped light waves out of thesubstrate, their impact on the optical performance of the LOE can beonly negative. Thus, if there is no overlapping between the reflectingsurfaces, then there will be inactive optical portions in the outputaperture of the system and “holes” will exist in the image. On the otherhand, the inactive portions of the reflecting surfaces are certainlyactive with respect to the waves from the external scene. In addition,the major axis orientation of two adjacent surfaces cannot be identical;otherwise the entire second surface will be inactive. Therefore, ifoverlapping is set between the reflective surfaces to compensate for theinactive portions in the output aperture then rays from the output scenethat cross these overlapped areas will suffer from double attenuationsand holes will be created in the external scene.

FIG. 11 illustrates a method of overcoming this problem. Only the activeportions of the partially reflecting surfaces are embedded inside thesubstrate, and since the reflecting surfaces are adjacent to oneanother, there will be no holes in the projected image, and since thereis no overlap between the surfaces there will be no holes in theexternal view. To achieve that, a blank plate 111 is attached,preferably by optical cementing, to the active area of the substrate,preferably by optical cementing.

In order to utilize the active areas of the reflective surfaces 22 onlyin the correct manner, it is important to calculate the actual activearea of each reflective surface. As illustrated in FIG. 12, the activeaperture, Dn, of the reflective surface 22 n in the plane of theexternal surface, as a function of the coupled-in angle α_(in), is:

$\begin{matrix}{D_{n} = {d \cdot {\frac{{\cot\left( \alpha_{sur} \right)} + {\cot\left( \alpha_{i\; n} \right)}}{2}.}}} & (17)\end{matrix}$

Since the trapped angle α_(in), can be varied as a function of the FOV,it is important to know with which angle to associate each reflectingsurface 22 n, in order to calculate its active aperture.

FIG. 13 illustrates the active aperture as a function of field angle fora system having the parameters: substrate thickness d=4 mm, substraterefractive index v=1.51, and reflective surface angle α_(sur)=64°.

In consideration of the viewing angles, it is noted that differentportions of the resulting image originate from different portions of thepartially reflecting surfaces.

FIG. 14 constitutes a sectional view of a compact LOE display systembased on the proposed configuration, illustrates this effect. Here, asingle plane wave 112, representing a particular viewing angle 114,illuminates only part of the overall array of partially reflectingsurfaces 22. Thus, for each point on the partially reflecting surface, anominal viewing angle is defined, and the required active area of thereflecting surface is calculated according to this angle.

The exact, detailed design of the active area of the various partiallyreflective surfaces is performed as follows: for each particularsurface, a ray is plotted (taking refraction, due to Snell's Law, intoconsideration) from the left edge of the surface to the center of thedesignated eye pupil 24. The calculated direction is set as the nominalincident direction and the particular active area is calculatedaccording to that direction. The exact values of the reflecting surfacesactive areas can be used to determine the various distances between thereflecting surfaces 22. A larger active area dictates a largerinter-surface distance. However, more accurate calculations should beperformed in order to determine exact distances between any two adjacentreflecting surfaces.

FIG. 15 illustrates this issue. As explained above, the projection ofeach surface is adjacent to its neighbor so as to avoid eitheroverlapping or gaps between the reflecting surfaces. However, this istrue for the central viewing angle only. For the right-most reflectingsurface, there are gaps 116 between the right-most surfaces, whereasthere is overlapping 118 between the left-most surfaces. Controlling thedistances between each pair of adjacent surfaces 22 can solve thisproblem. That is, the inter-surface distances will be smaller for theright surfaces and larger for the left surfaces. As a result, thiseffect partially compensates the divergence in surface distances, whichis described above with regards to active area sizes.

FIG. 16 illustrates the required distance between two adjacent surfacesas a function of the field angle for the same parameters as set above inreference to FIG. 13. As above, the detailed design of the distancebetween to adjacent reflecting surfaces is performed as follows: foreach particular surface, a ray is plotted (taking refraction, due toSnell's Law, into consideration) from the left edge of the surface tothe center of the designated eye 24. The calculated direction is set asthe nominal incident direction and the particular distance is calculatedaccording to that direction.

FIG. 17 illustrates an LOE 20 with reflecting surfaces 22 which havedifferent active apertures and different distances between the adjacentsurfaces accordingly. In order to achieve the required structure, awedged substrate 20, i.e., wherein the two major surfaces arenon-parallel, can be constructed. A complementary blank wedged plate 119is attached to the substrate, preferably by optical cementing, in such away that the combined structure from a complete rectangularparallelepiped. That is, the two outer major surfaces of the final LOEare parallel to each other. It is important to note that thecomplementary blank wedged plate can be utilized not only for LOE havinganisotropic reflecting surfaces but also for other types of LOEs,wherein all the partially reflective surfaces are exploiting isotropicangular sensitive thin film coatings. Usually, the geometry presented inFIG. 17 is required only for systems with a large number of facets orwhen the exact dimension of the overall output aperture is critical. Formost systems a simpler geometry, which is described in reference to FIG.11, may be sufficient. Considering, for example˜an optical system ofthree reflecting surfaces with the same parameters as described inreference to FIG. 12, and with an eye-relief (the distance between theeye of a viewer and the LOE) of R_(eye)=30 mm, the calculated activeareas of the surfaces are 6.53 nun, 5.96 mm and 5.45 mm. It is possibleto fabricate a much simpler LOE having apertures of 6 mm for all threesurfaces. The overall output aperture is smaller by 0.5 mm than for theoptimal configuration, but the fabrication process is much simpler. Theleftmost 0.5 mm of the third surface is not active, but this causes nointerference, since there is no overlapping there.

FIG. 18 illustrates a method of fabricating the required array ofpartially reflecting surfaces. First at step (a), a group of prisms 120and an associated group of anisotropic reflecting surfaces (mounted onthin plates) 122 are manufactured, having the required dimensions. Theprisms 120 can be fabricated from silicate-based materials such as BK-7with the conventional techniques of grinding and polishing, oralternatively, they can be made from polymer or sol-gel materials usinginjection molding or casting techniques. Finally at step (b), the prismsand the reflecting surface plates are glued together to form the desiredLOE 124. For applications in which the quality of the optical surfacesis critical, the final step (c) of polishing the outer surfaces 126 canbe added to the process. A modified version of this process can beperformed if the anisotropic reflecting surfaces 128 are fabricateddirectly onto the surfaces of the prisms (step (d)) and then the prismsare glued together to create the desired LOE 130 (step (e)).

FIG. 19 illustrates another method of fabricating an array of partiallyreflecting surfaces. A plurality of transparent flat plates 132 andanisotropic reflecting surfaces 134 step (a) are glued together so as tocreate a stacked form 136 step (b). A segment 138 step (c) is thensliced off the stacked form by cutting, grinding and polishing, tocreate the desired LOE 140 step (d). Several elements 142 can be slicedoff from this stacked form, as shown in (e).

FIG. 20 illustrates a modified method of fabricating an array ofpartially reflecting surfaces. Here the anisotropic reflecting surfaces144 at (a) are fabricated directly onto the surfaces of the plurality oftransparent flat plates, which are glued together so as to create astacked form 146 step (b). A segment 148 (c) is then sliced off thestacked form by cutting, grinding and polishing, to create the desiredLOE 150, as shown in (d).

FIG. 21 illustrates a method, applicable to each of the fabricationmethods described in reference to FIGS. 18 to 20 in which a blank plate152 step (a) is attached to one of the major surfaces of the substrate150, preferably using optical cement, so as to form an LOE 153 step (b)with the appropriate active apertures for all of the reflectingsurfaces. In order to materialize the LOE illustrated in FIG. 17, boththe substrate 150 and the blank plate 152 have a wedge structure. Inthat such a case it is usually required that the two external majorsurfaces, 154 and 155, are parallel to each other.

FIG. 22 illustrates yet another method of fabricating the array ofpartially reflecting surfaces. Two similar, tooth-shaped transparentforms 156 are fabricated step (a), by injection-molding or casting. Therequired anisotropic reflecting surfaces 158 are inserted in theappropriate places between the forms and the two forms are then step (b)glued together, to create the required LOE 160.

FIG. 23 illustrates yet another version of the method described in FIG.21 for fabricating the array of partially reflecting surfaces. Insteadof inserting the reflecting surfaces 158, the surfaces are applied atstep (a) to a very thin and flexible polymer sheet 162. Then, at step(b), the sheet 162 is inserted between forms 156, which are then gluedtogether to create the requested LOE.

So far it has been described how to fabricate the coupling-out activearea of the LOE. However, it should be noted that it is important notonly to couple the image out of the substrate without any distortion orghost image but also to couple the light waves properly into thesubstrate. FIG. 24, which illustrates one method for coupling-in,presents a sectional view of the reflective surface 16, which isembedded inside the substrate 20 and couples light waves 18 from adisplay source (not shown) and traps it inside the substrate 20 by totalinternal reflection. To avoid an image with gaps or stripes, it isessential that the trapped light cover the entire area of the LOE majorsurfaces. To ensure this, the points on the boundary line 170 betweenthe edge of the reflective surface 16 and the upper surface 26 a of thesubstrate 20 should be illuminated for a single wave by two differentrays that enter the substrate from two different locations; a ray 18 a,which illuminates the boundary line 170 directly, and another ray 18 b,which is first reflected by the reflecting surface 16 and then by thelower surface 26 b of the substrate, before illuminating the boundaryline. As illustrated in FIG. 25, the coupling-in substrate 171 can beattached step (a) at one of its peripheral sides to the coupling outsubstrate 154, to form step (b), a complete LOE form 20.

The embodiment described above with regards to FIG. 24 is an example ofa method for coupling input light waves into the substrate through oneof the major surfaces of substrate. Input light waves, can, however, becoupled into the substrate by other optical means as well, including(but not limited to) folding prisms, fiber optic bundles, diffractiongratings, and other techniques. Also, in the example illustrated in FIG.2, the input light waves and the image light waves are located on thesame side of the substrate. Other configurations are envisioned in whichthe input and the image light waves could be located on opposite sidesof the substrate.

In certain applications it is necessary to couple the input light wavesinto the substrate through one of the peripheral sides of the substrate.FIG. 26 illustrates a method of coupling light waves into the substratethrough one of its edges. Here, the light waves-transmitting substratehas two major parallel surfaces 26 and edges, wherein at least one edge172 is oriented at an oblique angle with respect to the major surfacesand wherein α_(edge) is the angle between the edge 172 and the normal tothe major surfaces of the substrate. Beside the substrate, the opticalmodule comprises an optical means for coupling light waves into saidsubstrate by internal reflection. In the example of FIG. 26, thisoptical means is a prism 174 wherein one of its surfaces 176 is locatednext to the said slanted edge 172 of the substrate. The prism alsocomprises two additional polished surfaces, 178 and 180. An optical ray182 enters the prism 174 through the surface 180, is reflected by totalinternal reflection off surface 176, then reflected off surface 178. Itthen enters the substrate 20 through the edge 172. The ray 182 is thentrapped inside the substrate 20 by total internal reflection. It is thencoupled out of the substrate by reflection off the reflecting surfaces22.

FIG. 27 illustrates an embodiment of the present invention, in which theLOE 20 is embedded in an eyeglass frame 188. The display source 4, thecollimating lens, and the folding element 190 are assembled inside thearm portions 192 of the eyeglass frames, next to the edge of the LOE 20.For a case in which the display source is an electronic element, such asa small CRT, LCD or OLED, the driving electronics 194 for the displaysource might be assembled inside the back portion of the arm 192. Apower supply and data interface 196 can be connected to arm 192 by alead 198 or any other communication means, including radio or opticaltransmission. Alternatively, a battery and miniature data linkelectronics can be integrated into the eyeglass frames.

The embodiment described above can serve in both see-through andnon-see-through systems. In the latter case, opaque layers are locatedin front of the LOE. It is not necessary to occlude the entire LOE, justthe active area, where the display is visible. In this way, the devicemaintains peripheral vision for the user, replicating the viewingexperience of a computer or a television screen, in which suchperipheral vision serves an important cognitive function. Alternatively,a variable filter can be placed in front of the system in such a waythat the viewer can control the level of brightness of the light wavesemerging from the external scene. This variable filter could either be amechanically controlled device, such as a folding filter or two rotatingpolarizers, an electronically controlled device, or even an automaticdevice whereby the transmittance of the filter is determined by thebrightness of the external background.

There are some alternatives as to the precise way in which an LOE can beutilized in this embodiment. The simplest option is to use a singleelement for one eye. Another option is to use an element and a displaysource for each eye, projecting the same image. Alternatively it ispossible to project two different parts of the same image, with someoverlap between the two eyes, enabling a wider FOV. Yet anotherpossibility is to project two different scenes, one to each eye, inorder to create a stereoscopic image. With this alternative, attractiveimplementations are possible, including 3-dimensional movies, advancedvirtual reality, training systems and others.

FIG. 28 illustrates a modified version of the embodiment described inFIG. 27. In addition to the components which are embedded in theeyeglass frames, a miniature video camera 200 with optional optical zoomcapability is installed in the front region of the frame 192. The cameracaptures images from the external scene, transfers the video signal toan image processing unit 202, which can be installed inside theelectronics unit 194 and which can be controlled in real-time by theuser. The processed image signal is then transferred to the image source4, which projects the image through the LOE 20 into the eye of the user.

The embodiment of FIG. 28 can be implemented in a wide variety ofapplications. A possible utilization is for users who require an abilityto perform close-up views on distant objects. Here the user can set thezoom position of the video camera according to the desiredmagnification. The captured image can then be processed and projected bythe optical system. Another application can combine a thermal camera ora miniature star-light-amplifier (SLA) to materialize a night-visiongoggle device. Here, the image from the external scene can be recorded,even in bad lighting conditions, and translated by the processing unit202 to a conventional video image which can be seen easily by the user.

Another potential application of the embodiment illustrated in FIG. 28is a visual aid for people who suffer from age-related maculardegeneration (AMD). AMD is a progressive eye condition affecting manymillions people around the world. The disease attacks the macula of theeye, where the sharpest central vision occurs. Although it rarelyresults in complete blindness, it destroys the clear, “straight ahead”central vision necessary for reading, driving, identifying faces,watching television, doing fine detailed work, safely navigating stairsand performing other daily tasks that are usually taken for granted,leaving only dim images or black holes at the center of vision. It canalso dim contrast sensitivity and color perception. Peripheral visionmay not be affected, and it is possible to see “out of the corner of theeye.”

Presently, there are some products in the market to assist with lowvision. One of the more popular devices is the spectacle-mountedmagnifiers which exploit the undamaged peripheral vision of a patientenabling functioning as normally as possible. Spectacle-mountedtelescopes for distance, or spectacle-mounted microscopes for close-up,can significantly improve visual capabilities. These devices protrudefrom the spectacle frame, and can be used with one or both eyes and canmagnify between 2 to 10 times, depending on the size of the telescope.As the desired magnification of the system increases, these devicesbecome larger, heavier and bulkier, and therefore, even for moderateperformance, are impractical. This is a major drawback for all kinds ofdisplays but especially in head-mounted applications and even more sofor elderly users, wherein the system must necessarily be as light andas compact as possible. Another disadvantage is the “unsocialappearance” of the device, resulting from its strange shape and largedimensions. In addition, the functionality with this device cansometimes be very complicated. For instance, when usingspectacle-mounted microscopes, objects must be held much closer to theeyes than normal. Since the embodiment described in FIG. 28 can be ascompact and light as conventional spectacles, with the same “naturallook”, this device can be a good candidate for use as an effectiveuser-friendly low vision aid for people who suffer from AMD. The usercan control the zoom of the optical system to achieve the requiredoptical magnification easily, in accordance with his medical conditionsand with the external scene. Moreover, this type of spectacles reflectsthe functionality of bifocals in that they allow a person to switch tothe required zoom operation for improved distance vision, and back tothe conventional spectacle lens for general orientation, with theadditional advantage that this zoom is dynamic and continuous.

The embodiments of FIGS. 27 and 28 are just examples illustrating thesimple implementation of the present invention. Since thesubstrate-guided optical element, constituting the core of the system,is very compact and lightweight, it could be installed in a vast varietyof arrangements. Hence, many other embodiments are also possible,including a visor, a folding display, a monocle, and many more. Thisembodiment is designated for applications where the display should benear-to-eye; head-mounted, head-worn or head-carried. There are,however, applications where the display is located differently. Anexample of such an application is a hand-held device for mobileapplication, such as for example a cellular phone. These devices areexpected to perform novel operations in the near future, which requirethe resolution of a large screen, including videophone, internetconnection, access to electronic mail, and even the transmission ofhigh-quality television satellite broadcasting. With the existingtechnologies, a small display could be embedded inside the phone,however, at present, such a display can project either video data ofpoor quality only, or a few lines of Internet or e-mail data directlyinto the eye.

FIG. 29 illustrates an alternative method, based on the presentinvention, which eliminates the current necessary compromise between thesmall size of mobile devices and the desire to view digital content on afull format display. This application is a hand-held display (HHD),which resolves the previously opposing requirements of achieving smallmobile devices, and the desire to view digital content on a full formatdisplay, by projecting high quality images directly into the eye of theuser. An optical module including the display source 4, a first displaysource, the folding and collimating optics 190 and the substrate 20 isintegrated into the body of a cellular phone 210, where the substrate 20replaces the existing protective cover-window of the phone.Specifically, the volume of the support components, including the firstdisplay source 4 and optics 190, is sufficiently small to fit inside theacceptable volume for modem cellular devices. In order to view the fullscreen, transmitted by the device, the window of the device ispositioned in front of the user's eye 24, observing the image with highFOV, a large eye-motion-box and a comfortable eye-relief. It is alsopossible to view the entire FOV at a larger eye-relief by tilting thedevice to display different portions of the image. Furthermore, sincethe optical module can operate in see-through configuration, a dualoperation of the device is possible; namely there is an option tomaintain the conventional cellular display 212, a second display source,intact. In this manner, the standard, low-resolution display can beviewed through the LOE 20 when the first display source 4 is shut-off.In a second, virtual-mode, designated for e-mail reading, internetsurfing, or video operation, the conventional second display source 212is shut-off, while the first display source 4 projects the required wideFOV image into the eye of the viewer through the LOE 20. The embodimentdescribed in FIG. 29 is only an example, illustrating that applicationsother than head-mounted displays can be materialized. Other possiblehand-carried arrangements include palm computers, compact entertainmentdevices like the iPod, small displays embedded into wristwatches, apocket-carried display having the size and weight reminiscent of acredit card, and many more. Alternatively, instead of integrating theLOE inside the HHD as illustrated in FIG. 29, it is clearly possible tofabricate a separate viewing device, as illustrated in FIGS. 27 and 28and connecting it into a conventional HHD. The viewing devicesillustrated in FIGS. 27 to 29 can be materialized not only by utilizedLOE having anisotropic reflecting surfaces but also by other types ofLOEs, wherein all the partially reflective surfaces are exploitingisotropic angular sensitive thin film coatings.

The embodiments described above are mono-ocular optical systems, thatis, the image is projected onto a single eye. There are, however,applications, such as head-up displays (HUD), wherein it is desired toproject an image onto both eyes. Until recently, HUD systems have beenused mainly in advanced combat and civilian aircraft. There have beennumerous proposals and designs, of late, to install a HUD in front of acar driver in order to assist in driving navigation or to project athermal image into his eyes during low-visibility conditions. Currentaerospace HUD systems are very expensive, the price of a single unitbeing of the order of hundreds of thousands of dollars. In addition, theexisting systems are very large, heavy, and bulky, and are toocumbersome for installation in a small aircraft let alone a car.LOE-based HUDs potentially provide the realization of a very compact,self-contained HUD, that can be readily installed into confined spaces.It also simplifies the construction and manufacturing of the opticalsystems related to the HUD and as such, could be suitable for improvingon aerospace HUDs, as well as introducing a compact, inexpensive,consumer version for the automotive industry.

FIG. 30 illustrates a method of materializing an HUD system based on thepresent invention. The light waves from a display source 4 arecollimated by a lens 6 to infinity and coupled by the first reflectingsurface 16 into substrate 20. After reflection at a second reflectingarray (not shown), the optical light waves impinge on a third set ofreflecting surfaces 22, which couple the light waves out into the eyes24 of the viewer. The overall system can be very compact andlightweight, of the size of a large postcard having a thickness of a fewmillimeters. The display source, having a volume of a few cubiccentimeters, can be attached to one of the corners of the substrate,where an electrical cord can transmit the power and data to the system.It is expected that the installation of the presented HUD system will beno more complicated than the installation of a simple commercial audiosystem. Moreover, since there is no need for an external display sourcefor image projection, the necessity to install components in unsafeplaces is avoided.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

The invention claimed is:
 1. A method for fabricating an optical deviceincluding at least one partially reflecting surface and a light-wavetransmitting substrate that allows light-waves to traverse thelight-wave transmitting substrate between external surfaces of thelight-wave transmitting substrate, the method comprising: obtaining aplurality of substantially transparent plates, each of the transparentplates having an associated thickness, and wherein the thickness of twoof the transparent plates is increased relative to the thickness of allremaining transparent plates of the plurality transparent plates;coating at least one surface of at least one of the transparent plates,other than the two transparent plates having increased thickness, with apartially reflecting coating to form the at least one partiallyreflecting surface; stacking the plates in a stack such that the twotransparent plates having increased thickness are at opposite ends ofthe stack, and attaching the stacked plates to each other; slicing thestacked plates to form the light-wave transmitting substrate such thatthe light-wave transmitting substrate includes at least a first and asecond major external surface; grinding or polishing the light-wavetransmitting substrate; and coating at least one of the first or secondmajor external surfaces with a coating to compensate for non-uniformityof the light-wave transmitting substrate.
 2. The method of claim 1,wherein the slicing the stacked plates includes: cutting the stackedplates along two parallel planes, and cutting the stacked plates alongat least one plane obliquely angled to the two parallel planes, suchthat the first and second major external surfaces are parallel to eachother and the light-wave transmitting substrate further includes atleast one slanted edge at an oblique angle with respect to the first andsecond major external surfaces.
 3. The method of claim 1, furthercomprising: attaching a blank plate to one of the first or second majorexternal surfaces.
 4. The method of claim 1, further comprising:attaching an optical element to the light-wave transmitting substratefor coupling light waves into the light-wave transmitting substrate bytotal internal reflection between the first and second major externalsurfaces.
 5. The method of claim 4, wherein the first and second majorexternal surfaces are parallel to each other and wherein the light-wavetransmitting substrate includes at least one slanted edge at an obliqueangle with respect to the first and second major external surfaces, andwherein the optical element is attached to the light-wave transmittingsubstrate at the at least one slanted edge.
 6. The method of claim 1,wherein the at least one partially reflecting surface is carried by thelight-wave transmitting substrate.
 7. The method of claim 1, wherein thecoating at least one surface of the at least one of the transparentplates includes applying the partially reflecting coating to a first anda second major surface of the at least one of the plates.
 8. The methodof claim 1, wherein the at least one partially reflecting surface isformed from at least one plate that is separate from the plurality ofplates.
 9. The method of claim 8, wherein the at least one plate that isseparate from the plurality of plates is formed from an angularsensitive thin film.
 10. The method of claim 1, wherein the at least onepartially reflecting surface includes a plurality of partiallyreflecting surfaces, and wherein at least two of the partiallyreflecting surfaces have different reflectivity.
 11. The method of claim10, wherein the non-uniformity of the light-wave transmitting substrateis caused by the different reflectivity of the at least two of thepartially reflecting surfaces.
 12. The method of claim 1, furthercomprising: coating at least one surface of at least one of thetransparent plates with a polarization sensitive coating.
 13. The methodof claim 1, wherein the thickness of the transparent plates that arecoated with the partially reflecting coating is decreased relative tothe thickness of all remaining transparent plates of the pluralitytransparent plates.
 14. A method for fabricating an optical deviceincluding a plurality of partially reflecting surfaces and a light-wavetransmitting substrate that allows light-waves to traverse thelight-wave transmitting substrate between external surfaces of thelight-wave transmitting substrate, the method comprising: obtaining aplurality of substantially transparent plates, each of the transparentplates having an associated thickness; coating at least one surface of asubset of the transparent plates with a partially reflecting coating toform a plurality of partially reflecting surfaces, wherein the thicknessof the transparent plates in the subset is decreased relative to thethickness of all remaining transparent plates of the pluralitytransparent plates; stacking the plates in a stack such that theplurality of partially reflecting surfaces is interleaved with thetransparent plates which are not coated with the partially reflectingcoating, wherein the thickness of two of the transparent plates whichare not coated with the partially reflecting coating is increasedrelative to the thickness of all remaining transparent plates of theplurality transparent plates and wherein the two of the transparentplates having increased thickness are at opposite ends of the stack;attaching the stacked plates to each other; slicing the stacked platesto form the light-wave transmitting substrate such that the light-wavetransmitting substrate includes at least a first and a second majorexternal surface; grinding or polishing the light-wave transmittingsubstrate; and coating at least one of the first or second majorexternal surfaces with a coating to compensate for non-uniformity of thelight-wave transmitting substrate.